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	<title>Tips on Writing Portable SQL for Multiple Databases for PHP</title>
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<table width=100% border=0><tr><td><h2>Tips on Writing Portable SQL &nbsp;</h2></td><td>
 <div align=right><img src="cute_icons_for_site/adodb.gif"></div></td></tr></table>

 If you are writing an application that is used in multiple environments and
  operating systems, you need to plan to support multiple databases. This article
  is based on my experiences with multiple database systems, stretching from 4th
  Dimension in my Mac days, to the databases I currently use, which are: Oracle,
  FoxPro, Access, MS SQL Server and MySQL. Although most of the advice here applies
  to using SQL with Perl, Python and other programming languages, I will focus on PHP and how
  the <a href="http://php.weblogs.com/adodb">ADOdb</a> database abstraction library
  offers some solutions.<p></p>
<p>Most database vendors practice product lock-in. The best or fastest way to
  do things is often implemented using proprietary extensions to SQL. This makes
  it extremely hard to write portable SQL code that performs well under all conditions.
  When the first ANSI committee got together in 1984 to standardize SQL, the database
  vendors had such different implementations that they could only agree on the
  core functionality of SQL. Many important application specific requirements
  were not standardized, and after so many years since the ANSI effort began,
  it looks as if much useful database functionality will never be standardized.
  Even though ANSI-92 SQL has codified much more, we still have to implement portability
  at the application level.</p>
<h3><b>Selects</b></h3>
<p>The SELECT statement has been standardized to a great degree. Nearly every
  database supports the following:</p>
<p>SELECT [cols] FROM [tables]<br>
  &nbsp;&nbsp;[WHERE conditions]<br>
  &nbsp; [GROUP BY cols]<br>
  &nbsp; [HAVING conditions] <br>
  &nbsp; [ORDER BY cols]</p>
<p>But so many useful techniques can only be implemented by using proprietary
  extensions. For example, when writing SQL to retrieve the first 10 rows for
  paging, you could write...</p>
<table width="80%" border="1" cellspacing="0" cellpadding="0" align="center">
  <tr>
	<td><b>Database</b></td>
	<td><b>SQL Syntax</b></td>
  </tr>
  <tr>
	<td>DB2</td>
	<td>select * from table fetch first 10 rows only</td>
  </tr>
  <tr>
	<td>Informix</td>
	<td>select first 10 * from table</td>
  </tr>
  <tr>
	<td>Microsoft SQL Server and Access</td>
	<td>select top 10 * from table</td>
  </tr>
  <tr>
	<td>MySQL and PostgreSQL</td>
	<td>select * from table limit 10</td>
  </tr>
  <tr>
	<td>Oracle 8i</td>
	<td>select * from (select * from table) where rownum &lt;= 10</td>
  </tr>
</table>
<p>This feature of getting a subset of data is so useful that in the PHP class
  library ADOdb, we have a SelectLimit( ) function that allows you to hide the
  implementation details within a function that will rewrite your SQL for you:</p>
<pre>$connection-&gt;SelectLimit('select * from table', 10);
</pre>
<p><b>Selects: Fetch Modes</b></p>
<p>PHP allows you to retrieve database records as arrays. You can choose to have
  the arrays indexed by field name or number. However different low-level PHP
  database drivers are inconsistent in their indexing efforts. ADOdb allows you
  to determine your prefered mode. You set this by setting the variable $ADODB_FETCH_MODE
  to either of the constants ADODB_FETCH_NUM (for numeric indexes) or ADODB_FETCH_ASSOC
  (using field names as an associative index).</p>
<p>The default behaviour of ADOdb varies depending on the database you are using.
  For consistency, set the fetch mode to either ADODB_FETCH_NUM (for speed) or
  ADODB_FETCH_ASSOC (for convenience) at the beginning of your code. </p>
<p><b>Selects: Counting Records</b></p>
<p>Another problem with SELECTs is that some databases do not return the number
  of rows retrieved from a select statement. This is because the highest performance
  databases will return records to you even before the last record has been found.
</p>
<p>In ADOdb, RecordCount( ) returns the number of rows returned, or will emulate
  it by buffering the rows and returning the count after all rows have been returned.
  This can be disabled for performance reasons when retrieving large recordsets
  by setting the global variable $ADODB_COUNTRECS = false. This variable is checked
  every time a query is executed, so you can selectively choose which recordsets
  to count.</p>
<p>If you prefer to set $ADODB_COUNTRECS = false, ADOdb still has the PO_RecordCount(
  ) function. This will return the number of rows, or if it is not found, it will
  return an estimate using SELECT COUNT(*):</p>
<pre>$rs = $db-&gt;Execute(&quot;select * from table where state=$state&quot;);
$numrows = $rs-&gt;PO_RecordCount('table', &quot;state=$state&quot;);</pre>
<p><b>Selects: Locking</b> </p>
<p>SELECT statements are commonly used to implement row-level locking of tables.
  Other databases such as Oracle, Interbase, PostgreSQL and MySQL with InnoDB
  do not require row-level locking because they use versioning to display data
  consistent with a specific point in time.</p>
<p>Currently, I recommend encapsulating the row-level locking in a separate function,
  such as RowLock($table, $where):</p>
<pre>$connection-&gt;BeginTrans( );
$connection-&gt;RowLock($table, $where); </pre>
<pre><font color=green># some operation</font></pre>
<pre>if ($ok) $connection-&gt;CommitTrans( );
else $connection-&gt;RollbackTrans( );
</pre>
<p><b>Selects: Outer Joins</b></p>
<p>Not all databases support outer joins. Furthermore the syntax for outer joins
  differs dramatically between database vendors. One portable (and possibly slower)
  method of implementing outer joins is using UNION.</p>
<p>For example, an ANSI-92 left outer join between two tables t1 and t2 could
  look like:</p>
<pre>SELECT t1.col1, t1.col2, t2.cola <br>  FROM t1 <i>LEFT JOIN</i> t2 ON t1.col = t2.col</pre>
<p>This can be emulated using:</p>
<pre>SELECT t1.col1, t1.col2, t2.cola FROM t1, t2 <br>	   WHERE t1.col = t2.col
   UNION ALL
SELECT col1, col2, null FROM t1 <br>	   WHERE t1.col not in (select distinct col from t2)
</pre>
<p>Since ADOdb 2.13, we provide some hints in the connection object as to legal
  join variations. This is still incomplete and sometimes depends on the database
  version you are using, but is useful as a general guideline:</p>
<p><font face="Courier New, Courier, mono">$conn-&gt;leftOuter</font>: holds the
  operator used for left outer joins (eg. '*='), or false if not known or not
  available.<br>
  <font face="Courier New, Courier, mono">$conn-&gt;rightOuter</font>: holds the
  operator used for right outer joins (eg '=*'), or false if not known or not
  available.<br>
  <font face="Courier New, Courier, mono">$conn-&gt;ansiOuter</font>: boolean
  that if true means that ANSI-92 style outer joins are supported, or false if
  not known.</p>
<h3><b>Inserts</b> </h3>
<p>When you create records, you need to generate unique id's for each record.
  There are two common techniques: (1) auto-incrementing columns and (2) sequences.
</p>
<p>Auto-incrementing columns are supported by MySQL, Sybase and Microsoft Access
  and SQL Server. However most other databases do not support this feature. So
  for portability, you have little choice but to use sequences. Sequences are
  special functions that return a unique incrementing number every time you call
  it, suitable to be used as database keys. In ADOdb, we use the GenID( ) function.
  It has takes a parameter, the sequence name. Different tables can have different
  sequences. </p>
<pre>$id = $connection-&gt;GenID('sequence_name');<br>$connection-&gt;Execute(&quot;insert into table (id, firstname, lastname) <br>			   values ($id, $firstname, $lastname)&quot;);</pre>
<p>For databases that do not support sequences natively, ADOdb emulates sequences
  by creating a table for every sequence.</p>
<h3><b>Binding</b></h3>
<p>Binding variables in an SQL statement is another tricky feature. Binding is
  useful because it allows pre-compilation of SQL. When inserting multiple records
  into a database in a loop, binding can offer a 50% (or greater) speedup. However
  many databases such as Access and MySQL do not support binding natively and
  there is some overhead in emulating binding. Furthermore, different databases
  (specificly Oracle!) implement binding differently. My recommendation is to
  use binding if your database queries are too slow, but make sure you are using
  a database that supports it like Oracle. </p>
<p>ADOdb supports portable Prepare/Execute with:</p>
<pre>$stmt = $db-&gt;Prepare('select * from customers where custid=? and state=?');
$rs = $db-&gt;Execute($stmt, array($id,'New York'));</pre>
<h2>DDL and Tuning</h2>
There are database design tools such as ERWin or Dezign that allow you to generate data definition language commands such as ALTER TABLE or CREATE INDEX from Entity-Relationship diagrams.
<p>
However if you prefer to use a PHP-based table creation scheme, adodb provides you with this feature. Here is the code to generate the SQL to create a table with:
<ol>
	<li> Auto-increment primary key 'ID', </li>
	<li>The person's 'NAME' VARCHAR(32) NOT NULL and defaults to '', </li>
	<li>The date and time of record creation 'CREATED', </li>
	<li> The person's 'AGE', defaulting to 0, type NUMERIC(16). </li>
</ol>
<p>
Also create a compound index consisting of 'NAME' and 'AGE':
<pre>
$datadict = <strong>NewDataDictionary</strong>($connection);
$flds = "
<font color="#660000">  ID I AUTOINCREMENT PRIMARY,
  NAME C(32) DEFAULT '' NOTNULL,
  CREATED T DEFTIMESTAMP,
  AGE N(16) DEFAULT 0</font>
";
$sql1 = $datadict-><strong>CreateTableSQL</strong>('tabname', $flds);
$sql2 = $datadict-><strong>CreateIndexSQL</strong>('idx_name_age', 'tabname', 'NAME,AGE');
</pre>

<h3>Data Types</h3>
<p>Stick to a few data types that are available in most databases. Char, varchar
  and numeric/number are supported by most databases. Most other data types (including
  integer, boolean and float) cannot be relied on being available. I recommend
  using char(1) or number(1) to hold booleans. </p>
<p>Different databases have different ways of representing dates and timestamps/datetime.
  ADOdb attempts to display all dates in ISO (YYYY-MM-DD) format. ADOdb also provides
  DBDate( ) and DBTimeStamp( ) to convert dates to formats that are acceptable
  to that database. Both functions accept Unix integer timestamps and date strings
  in ISO format.</p>
<pre>$date1 = $connection-&gt;DBDate(time( ));<br>$date2 = $connection-&gt;DBTimeStamp('2002-02-23 13:03:33');</pre>
<p>We also provide functions to convert database dates to Unix timestamps:</p>
<pre>$unixts = $recordset-&gt;UnixDate('#2002-02-30#'); <font color="green"># MS Access date =&gt; unix timestamp</font></pre>
<p>The maximum length of a char/varchar field is also database specific. You can
  only assume that field lengths of up to 250 characters are supported. This is
  normally impractical for web based forum or content management systems. You
  will need to be familiar with how databases handle large objects (LOBs). ADOdb
  implements two functions, UpdateBlob( ) and UpdateClob( ) that allow you to
  update fields holding Binary Large Objects (eg. pictures) and Character Large
  Objects (eg. HTML articles):</p>
<pre><font color=green># for oracle </font>
$conn->Execute('INSERT INTO blobtable (id, blobcol) VALUES (1,empty_blob())');
$conn->UpdateBlob('blobtable','blobcol',$blobvalue,'id=1');

<font color=green># non-oracle databases</font>
$conn->Execute('INSERT INTO blobtable (id, blobcol) VALUES (1, null)');
$conn->UpdateBlob('blobtable','blobcol',$blobvalue,'id=1');
</pre>
<p>Null handling is another area where differences can occur. This is a mine-field,
  because 3-value logic is tricky.
<p>In general, I avoid using nulls except for dates and default all my numeric
  and character fields to 0 or the empty string. This maintains consistency with
  PHP, where empty strings and zero are treated as equivalent, and avoids SQL
  ambiguities when you use the ANY and EXISTS operators. However if your database
  has significant amounts of missing or unknown data, using nulls might be a good
  idea.
<h3><b>Stored Procedures</b></h3>
<p>Stored procedures are another problem area. Some databases allow recordsets
  to be returned in a stored procedure (Microsoft SQL Server and Sybase), and
  others only allow output parameters to be returned. Stored procedures sometimes
  need to be wrapped in special syntax. For example, Oracle requires such code
  to be wrapped in an anonymous block with BEGIN and END. Also internal sql operators
  and functions such as +, ||, TRIM( ), SUBSTR( ) or INSTR( ) vary between vendors.
</p>
<p>An example of how to call a stored procedure with 2 parameters and 1 return
  value follows:</p>
<pre>	switch ($db->databaseType) {
	case '<font color="#993300">mssql</font>':
	  $sql = <font color="#000000"><font color="#993333">'<font color="#993300">SP_RUNSOMETHING</font>'</font></font>; break;
	case '<font color="#993300">oci8</font>':
	  $sql =
<font color="#993300">	  </font><font color="#000000"><font color="#993300">&quot;declare RETVAL integer;begin :RETVAL := </font><font color="#000000"><font color="#993333"><font color="#993300">SP_RUNSOMETHING</font></font></font><font color="#993300">(:myid,:group);end;&quot;;
</font>	  break;</font>
	default:
	  die('<font color="#993300">Unsupported feature</font>');
	}
<font color="#000000"><font color="green">	# @RETVAL = SP_RUNSOMETHING @myid,@group</font>
	$stmt = $db-&gt;PrepareSP($sql);	<br>	$db-&gt;Parameter($stmt,$id,'<font color="#993300">myid</font>');
	$db-&gt;Parameter($stmt,$group,'<font color="#993300">group</font>');
	<font color="green"># true indicates output parameter<br>	</font>$db-&gt;Parameter($stmt,$ret,'<font color="#993300">RETVAL</font>',true);
	$db-&gt;Execute($stmt); </font></pre>
<p>As you can see, the ADOdb API is the same for both databases. But the stored
  procedure SQL syntax is quite different between databases and is not portable,
  so be forewarned! However sometimes you have little choice as some systems only
  allow data to be accessed via stored procedures. This is when the ultimate portability
  solution might be the only solution: <i>treating portable SQL as a localization
  exercise...</i></p>
<h3><b>SQL as a Localization Exercise</b></h3>
<p> In general to provide real portability, you will have to treat SQL coding
  as a localization exercise. In PHP, it has become common to define separate
  language files for English, Russian, Korean, etc. Similarly, I would suggest
  you have separate Sybase, Intebase, MySQL, etc files, and conditionally include
  the SQL based on the database. For example, each MySQL SQL statement would be
  stored in a separate variable, in a file called 'mysql-lang.inc.php'.</p>
<pre>$sqlGetPassword = '<font color="#993300">select password from users where userid=%s</font>';
$sqlSearchKeyword = &quot;<font color="#993300">SELECT * FROM articles WHERE match (title,body) against (%s</font>)&quot;;</pre>
<p>In our main PHP file:</p>
<pre><font color=green># define which database to load...</font>
<b>$database = '<font color="#993300">mysql</font>';
include_once(&quot;<font color="#993300">$database-lang.inc.php</font>&quot;);</b>

$db = &amp;NewADOConnection($database);
$db->PConnect(...) or die('<font color="#993300">Failed to connect to database</font>');

<font color=green># search for a keyword $word</font>
$rs = $db-&gt;Execute(sprintf($sqlSearchKeyWord,$db-&gt;qstr($word)));</pre>
<p>Note that we quote the $word variable using the qstr( ) function. This is because
  each database quotes strings using different conventions.</p>
<p>
<h3>Final Thoughts</h3>
<p>The best way to ensure that you have portable SQL is to have your data tables designed using
sound principles. Learn the theory of normalization and entity-relationship diagrams and model
your data carefully. Understand how joins and indexes work and how they are used to tune performance.
<p> Visit the following page for more references on database theory and vendors:
  <a href="http://php.weblogs.com/sql_tutorial">http://php.weblogs.com/sql_tutorial</a>.
  Also read this article on <a href=http://phplens.com/lens/php-book/optimizing-debugging-php.php>Optimizing PHP</a>.
<p>
<font size=1>(c) 2002 John Lim.</font>

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